The invention relates to a display tube having an envelope, comprising a phosphor screen on one side and a neck portion on the other side, and an electron gun positioned in the neck portion and comprising a beam-shaping part for producing an electron beam, and a focusing structure for focusing the produced electron beam on the phosphor screen. When designing electron guns for display tubes, one of the problems of realizing a gun is a small deflection defocusing accompanied by a small spherical aberration. Hitherto it has always been tried to find a solution for the one aspect, while the other aspect was taken for granted.
For example, in U.S. Pat. No. 4,366,419 a color television display tube is described which comprises an electron gun of the in-line type having three individual focusing lenses each comprising a first and a second tubular electrode. The first electrode has means (diametrically facing transversal apertures each co-operating with an auxiliary electrode) for forming a non-rotationally symmetrical (astigmatic) lens element in the region of the first electrode. In this case voltages are applied to the electrodes such that the astigmatism and the power of the focusing lens are simultaneously controlled. In this way the deflection defocusing of the spot is combated, which defocusing is particularly intolerable in high-resolution color television display tubes. A drawback of the construction of the electron gun of the color television display tube described in the U.S. Pat. No. 4,366,419 is, however, that three metal tubular electrode sets have to be accommodated side by side in the neck of the tube so that the diameters of these metal tubular electrode sets are bound to maximum dimension, which means that the spot size as such cannot be very small as a result of spherical aberration, although the deflection defocusing is efficiently combated by providing a non-rotationally symmetrical electrically controlled lens element so that the spot size does not notably increase in the case of deflection.